Richard,


Is your fuel tank mounted so its center-line (not nipple/outlet/pipe) is at the exact same level as the carburettor's spray-bar, with the model in level flight attitude?

If it is, the engine will get the same mixture when maneuvering level and inverted.

If not, there's your problem.
Most ARF models will not have the fuel tank/engine combo properly set up... It is up to you to side-mount the engine to get these items properly leveled.

...Do these guys ever try flying the models they manufacture?


Also, make sure the tank is fully isolated from any and all airframe parts, using 1/4"+ latex rubber foam,to make sure no fuel-frothing affects your engine.
That includes the fuel outlets not touching anything solid.